1. Field of the Invention
This invention relates to a method and apparatus of combusion for a pipestill heater adapted to use the heat of the exhaust gas resulting from combustion for heating the oxidizing agent for combustion.
2. Description of the Prior Art
The pipestill heater is used in various processes pertaining to petroleum refining, petrochemistry, and general chemistry for the purpose of heating with an open flame a fluid subjected to heating which is flowing through tubes of the furnace.
The conventional pipestill heater, as illustrated in FIG. 8, comprises a combustion chamber 100 formed by lining a casing of steel plates with refractory insulating material, heating tubes 101 laid out inside the combustion chamber 100, and combustion devices 102 set in place therein. It is so configured as to heat such a fluid as naphtha or gasoline flowing in the heating tubes 101 by means of the combustion devices 102.
More often than not, the pipestill heater has heretofore effected the required heating by virtue of the radiation and convection of a flame. The portion of this furnace which is heated mainly by the radiative heat transfer forms a radiation section 103 and the portion thereof which is heated mainly by the convective heat transfer a convection section 104. The fluid subjected to heating is generally supplied in a direction opposite to the stream of the combustion gas from the viewpoint of exalting the thermal efficiency. The individual heating tubes 101 are connected in series with U-shaped joints to form what is called a coil path. The fluid subjected to heating is first introduced via an inlet tube 106 into the convection section 104 to be preheated therein, then advanced into the radiation section 103 to be heated to a prescribed temperature therein, and thereafter allowed to flow out of an outlet tube 107. In FIG. 8, the reference numeral "105" stands for a stack.
The thermal efficiency of the pipestill heater of this kind is generally in the range of 60 to 85%. Even with the pipestill heater of such a large size as illustrated in FIG. 9 (wherein like component members found in FIG. 8 are denoted by like reference numerals), it is difficult to obtain such a high thermal efficiency as 90% on account of the restriction imposed by the inlet temperature of the fluid on the preheating with the convection section 104. It is, therefore, conceivable to provide the pipestill heater under discussion with such an additional component as preheater 108 for preheating an oxidizing agent consisted of the air for combustion and the like. or a waste-heat boiler and exalt the thermal efficiency of the pipestill heater to a level above 90%. The words "oxidizing agent" as used herein constitute themselves the generic term for designating such gases as pure oxygen, air and oxygen-enriched air which contain molecular oxygen. In some case a halogen, a oxidizing material or chemical compound such as a nitric oxide may be used as an oxidizing agent. In FIG. 9, the reference numeral "109" stands for a forced draft fan and the reference numeral "110" for an induced draft fan.
In the pipestill heater of this kind, the amount of heat absorbed by the convention section 104 is less than half of the amount absorbed by the radiation section 103 in spite of the fact that the tube surface area of the convection section 104 available for heat absorption is more than twice that of the radiation section 103. In respect of the return of investment, some if not all small pipestill heaters omit the convention section 104 and resort solely to the radiation section 103. These furnaces which are devoid of the convection section 104, therefore, possibly suffer from inferior thermal efficiency due to insufficient preheating.
Some of the pipestill heaters such as, a furnace for use in a catalytic reforming plant necessitate absorption of a large amount of heat as a whole and yet, by reason of process, require to set the inlet temperature of the fluid subjected to heating at a high level (in the neighborhood of 440.degree. C.) and, what is more, the allowable pressure drop through the coil at an extremely low level (0.2 to 0.3 Kg/cm.sup.2). These pipestill heaters are at a disadvantage in inevitably imposing a limit of its own on the thermal efficiency to be attained at all and failing to acquire improvement in quality because the fluid supplied to the radiation section 103 to be heated therein cannot be likewise supplied to the convection section 104 and this fluid must be heated solely by the radiation section 103.
Further, the large pipestill heater of the kind illustrated in FIG. 9 is provided with such an additional component as the oxidizing agent preheater 108 or waste-heat boiler and, therefore, requires an increase in the length L.sub.2 in the direction of width of the furnace and not in the length L.sub.1 in the direction of length thereof and results in having a large floor space for the installation of the furnace. Moreover, such additional component as the oxidizing agent preheater 108 involves as high cost of construction as the furnace proper and results in boosting the overall cost of the pipestill heater facilities.